Electric variable condenser



y 1935. .P. GRASSMANN 2,001,459

ELECTRIC VARIABLE CONDENSER Filed Oct. 24, 1933 Patented May 14, 1935UNI-TED PATENT OFFICE This invention relates to electric variablecondensers and more particularly to condensers of the type which arecommonly employed in radio apparatus, usually for tuning purposes.

B An object of this invention is to provide a variable tuning condenserwhich will be of maximum electrical capacity while occupying a minimumamount of space. Another object is to provide a variable condenserembodying improved 10- means for accurately and uniformly spacing thevanes thereof.

These and other objects which will be more apparent hereinafter areattained in accordance with this invention by making both the fixed andmovable condenser vanes of extreme thinness, so thin as to be non-rigidand incapable of supporting their own weight, and mounting the vanes ofeach set in a rigid frame in which the vanes are tensloned so as to liein parallel planes. By 80- this means it is possible to reduce thespacing between the vanes to a minimum and this, together with theextreme thinness of the vanes, makes practicable a large capacitycondenser of minimum dimensions. Compact condensers of midgetproportions, which are extremely accurate in spite of their smallness,can thus be constructed. and this towards reducing the size andbulkiness of radio sets.

For a clearer understanding of the invention, one specific embodimentthereof will be described in the following, reference being bad to theaccomlllnylng drawing. in which Fig. 1 illustrates a section on the lineA-B of Figure 2, which illustrates in plan view a preferred form ofcondenser embodying the invention. The position of the set of movingplates in the position of minimum capacity is shown in chain-dottedlines. v 4

Referring to the drawing, the between 40 the vanes l and the thickn ofthe latter as represented in Pig. 1 are merely aticandarenotintendedtoshowtheactualdimensicns empl y d. Also, while Fig. 1,being a section on line A-B of Fig. 2, illustrates the set of 48flxedvanes,the-set of moving vanesmaybesimilarly represented.

Thevaneslconsistofeltremelythinmetal wafers or foils, for instance, 0.05mm. in thickness, and preferably of hard metal, for instance.copperorbrass. Thevanesarespacedbymetallicspacingwashers!of,forinstance,0.5mm. thickness. Inthe drawing,asetof twelve vanes is illustrated as an example. with thedimenispractically only6.i mm.

sions given, the overall depth of the entire set,

August 10,1932 5 (Cl. 175-415) The fixed and moving set of vanes aremounted in tensioning frames 4 and lrespectively. These frames aresimilarly'constructed and therefore a detailed description will be givenonly in connection with the frame 4 for the fixed vanes, it 5 beingunderstood that frame 1 is similar except in respect of such details aswill hereinafter be specifically mentioned.

The frame 4 corresponds in outline to the final shape or configurationof the vanes I which in the construction illustrated are in the form ofright-angle triangles, the sides of which are of unequal length. I havefound by experiment that this is one of the most suitable shapes.A1so,'in the construction illustrated in Fig. 2, the frame 4 isconstructed as a double frame. The upper and lower members of the frame4are connected together by the rods 3 located at the'angles of thetriangular-members. The rods 3 are hollow in order to facilitate thefixture of the set of fixed vanes to a casing and at the same time tofacilitate the connection of a number of condensets in series. Ofcourse, the fixing of the con-- denser in the casing may be effected ina different manner; for instance, by means of lugs or feet provided onthe frame 4.

Each condenser vane is stamped with its holes, for mounting on the rods3, under simultaneous plane-pressure, the holes being shaped such thatafter the vane and spacing washers 2 have been mounted in position onthe rods I of the frame 4, tensioning or stretching of the vane so as tomake it assume a plane surface is still possible.

In practice, it is suflicient for this purpose to make the holes in thevanes somewhat larger than the diameter of the rods I, so that the vanescan be looseLv mounted on the latter.

In the case of the fixed vane assembly, spacing bushes 6' (see Fig. 1)are. provided between the end vanes I and the triangular members of theframe 4 in order to provide the necessary clearance space for thepassage of the triangular members of the frame 1 carrying the 'movingvanes. The moving vane assembly on the frame 1 will, of course, notinclude bushes similar to the bushes 6 of the fixed frame assembly.

The fixed and moving vanes and also their frames form, as can be seenfrom the drawing; equal triangles, which substantially simplifiesmanufacture. Each frame has a bracket 0 which extends along its shortestedge and which, together with the hollow'spindle 9, forms the bearingfor the moving frame I and vanes. In this case, the frame 4 is insulatedfrom the frame I and the spindle O by insulating bushes II. The 56spindle 9 is firmly connected, in a manner not shown in detail, to thebrackets 8 of the frame I and may be combined with either an operatingknob or transmission gear, or with the spindle of an adjacent condenser,for ganging purposes. It will be noted that the spindle 9 is located ata point outside the effective surface of the moving vanes.

In the drawing, the rotating knob drive for the condenser which is fixedby means of screws to a base plate, is illustrated.

In Fig. 2, the position of the set of moving vanes at minimum capacityis represented by the chain-dotted lines. It will be seen that movementof the moving vanes completely from between the fixed vanes is preventedby the presence of the frames, so that a certain small initia orresiduaPcapacity arising from the overlapping cannot be avoided. Inorder to reduce this initial capacity, according to a feature of thisinvention, both the fixed and movable vanes are provided With recesses Il at the angles which overlap and produce the initial capacity,positioned and proportioned such that, in the positio'n of minimumcapacity, these recesses cross one another and thus only punctiformparts of the plates are opposite one another.

While it is preferable to use metallic frames, I do not wish to excludethe use of frames of insulating material. In the latter case, theinsulating bushes in become unnecessary.

The tensioning of the foils may be done in any suitable manner; forinstance by means of a tool in which the lugs of the vanes are clamped.The

tool may either be a tong-like clamping device provided with spacingmembers corresponding to the spacing members 2, or it may be a threadedbolt provided with similar spacing members between which the vanes areclamped and then tensioned all at the same time, that is to say, firstat one corner of the triangle and then at the second and third corner.When one side of the triangle has been straightened out, the nuts aretightened and the procedure is repeated at the second and third corners.Naturally, the first corner (preferably the one having the 90 angle) maybe clamped at once, for which reason the tensioning has to be done onlyfrom the two acute-angled corners.

I claim:-

1. An electric variable condenser comprising two rigid frames so mountedas to permit one to rotate within the other, and condenser vanes mountedin each of said frames, said vanes being so thin as to be non-rigid andincapable of supporting their own weight and being in stretchedcondition so as to lie in parallel, planes within said frames.

2. An electric variable condenser comprising two rigid frames-so mountedas to permit one to rotate within the other, and condenser vanes mountedin each of said frames, said frames and vanes being so shaped andproportioned that in the position of minimum capacity small areas ofsaid fixed and said movable vanes overlap, andmeans for reducing theminimum capacity due to said overlapping surfaces, said vanes being sothin as to be non-rigid and incapable of supporting their own weight andbeing in stretched condition so as to lie in parallel planes within saidframes.

3. An electric variable condenser comprising two rigid frames so mountedas to permit one to rotate within the other, and condenser vanes mountedin each of said frames, said frames and vanes being so shaped andproportioned that in the position of minimum capacity small. areas ofsaid fixed and said movable vanes overlap, and slots in said vanes atsaid overlapping areas, so positioned and shaped as to lie across oneanother and thereby reducethe minimum capacity due to said overlappingsurfaces, said vanes being so thin as to be non-rigid and incapable ofsupporting their own weightand being in stretched condition so as to liein parallel planes within said frames.

4. In the manufacture of an electric variable condenser, the steps ofmounting a non-rigid wafer-like condenser vane on a rigid frame formingpart of the condenser assembly, stretching said vane until it assumesthe form of a plane surface, and clamping said vane in such stretchedcondition.

5. An electric variable condenser according to claim 1, characterized inthat the said frames are of right-angle triangular form, one of which isrotatable with respect to the other about an axis lying adjacent butspaced from the right angle of the frame and outside the effectivesurfaces of the said condenser vanes.

PETER GRASSMANN.

